In recent years, variable resistance nonvolatile memory elements made using, as a storage material, a variable resistance material including a transition metal oxide which is oxygen-deficient compared to a transition metal oxide having stoichiometric composition have been proposed. Such a nonvolatile memory element includes an upper electrode layer, a lower electrode layer, and a variable resistance layer between the upper electrode layer and the lower electrode layer. Resistance of the variable resistance layer reversibly changes upon application of an electrical pulse between the upper electrode layer and the lower electrode layer. Information can be stored in the nonvolatile memory element in a non-volatile manner by associating the information with the values of the resistance (see Japanese Unexamined Patent Application Publication No. 2007-235139, for example). Such variable resistance nonvolatile memory elements are expected to be small and fast and consume a small amount of power compared to flash memories having floating gates.
However, there is a problem that the conventional variable resistance nonvolatile memory elements have characteristics which vary more widely than an expectation based on thickness and composition of variable resistance layers and electrodes and dimensions and configurations of photoresist masks after lithography or configurations of variable resistance layers and electrodes after dry etching. This leads to a problem that a larger-size nonvolatile memory device has poor retention or inappropriately changes in resistance in bits having the poorest characteristics (tail bits) due to variation among nonvolatile memory elements.